Turn classoption in style-config into list

See merge request hos/modernes-publizieren/interner-kreis/software/configurations/journal-configuration!1

Pandoc uses a new, stricter YAML library since version 2.8. For fields
defined multiple times in a block, the old version used the last value,
while the new version rejects the whole block as invalid. This caused
the pandoc-scholar to ignore the contents of `style-metadata.inc.yml`.